1. Field of the Invention
The invention relates to a backscatter transponder for an HF communication system with modulated reradiation.
The invention resides in the field of transponder technology and more particularly in the field of contactless communication for the purposes of identification. Although applicable in principle to any desired communication systems, the present invention and the problems it was designed to solve are explained below with respect to RFID communication systems and their applications. RFID stands for “Radio Frequency Identification.”
2. Description of the Background Art
In RFID systems, typically an electromagnetic signal sent out by a base station (reader) is received and demodulated by the transponder (tag). In this context, a distinction is made between active, semi-passive, and passive transponders, depending on how their energy supply is implemented. In contrast to active transponders, passive transponders have no energy supply of their own, so the energy required in the transponder for demodulation and decoding of the received electromagnetic signal must be extracted from the very electromagnetic signal transmitted by the base station. In addition to this unidirectional transfer of energy, bi-directional data communication between the base station and transponder typically also takes place. For general background information on RFID technology, refer to the “RFID Handbuch” by Klaus Finkenzeller, Hanser Verlag, third revised edition, 2002, which was translated in English by John Wiley & Sons.
In most UHF and microwave based RFID systems or sensor systems, bidirectional data communication between base station and transponder is first initiated by the base station in that the base station transmits a query signal (command, data request) to the various transponders located in the vicinity of the base station. The transponder or transponders participating in the data communication typically react to this query with a response signal (response).
The method known as the backscatter technique is one method used for return data transmission from the transponder back to the base station. Such transponders are referred to below as backscatter transponders. In this method, first the base station emits high-frequency electromagnetic carrier signals, which are received and processed by the transmitting and receiving device in the transponder. In addition, the received signals are modulated with a customary modulation method and are scattered back using the backscatter cross-section of the transponder's transmitting/receiving antenna.
European patent EP 750 200 B1, which corresponds to U.S. Pat. No. 5,649,295, describes a prior art RFID communication system in which the return data transmission takes place with the use of the backscatter method. In this system, the base station transmits a first piece of information to at least one transponder of the communication system, the transponder(s) receive the signal transmitted by the base station, decode it, and take the first piece of information from it. The transponders also have a backscatter modulator; the backscatter modulator modulates the backscattered signal, which is derived from the signal transmitted by the base station. This modulation uses a second piece of information, which is derived from the first piece of information of the transmitted signal. The content of the second piece of information in the backscattered signal includes the data rate or the modulation for the signals backscattered by the transponder, for example.
At times, there is a need for a rapid and maximally effective test capability for such transponders, in order to thereby determine one or more transponder-specific parameters that can provide information concerning the quality, performance, functionality, etc. of the transponder. A very important transponder-specific parameter, which provides information about the overall RFID system in addition to the transponder itself, is the effectiveness of the transponder's rectifier. To a first approximation, the effectiveness of the rectifier describes the ratio of the useful signal picked up by the transponder to the transmit signal transmitted by the base station. Since only an indication of the losses in the transmission path of the electromagnetic wave is typically obtained in this way, it is of course necessary to also take the ratio of the DC power at the output of the rectifier to the useful signal picked up in order to determine the actual rectifier effectiveness.
If the transponder or the rectifier has many parasitic components on the receive side, the effectiveness of the rectifier drops. It is important to be able to identify precisely these transponder-specific parameters as accurately as possible in order to evaluate the quality and performance of a given transponder.
Additional transponder-specific parameters include the modulation amplitude of the modulator, which is a function of the voltage extracted by the transponder from the electromagnetic field and which significantly influences the backscatter behavior, the voltage extracted from the external electromagnetic field, the receive quality, and the like.
These transponder-specific parameters of a UHF-based or microwave-based RFID systems can be determined in any of several ways.
The transponder-specific parameters may be determined statically, for example. In this method, the transponder-specific parameter or parameters desired in each case are measured and analyzed by the transponder itself. The analyzed parameters are then written to a register or memory internal to the transponder. Subsequent to the testing, the stored transponder-specific parameters can be read out by an external reader device. However, this method is extraordinarily expensive, since it is necessary to provide a transponder specifically intended for this purpose which has appropriate analysis and storage devices. Since the passive and semipassive transponders that are customarily used do not have transponder-internal test devices for the sole purpose of testing, not least for reasons of cost, transponders so equipped would be complicated in terms of circuit design, and thus cost-intensive. Furthermore, only a static test would be possible with this type of transponder test, since the data acquired could only be read out and analyzed subsequent to the test. The option of dynamic testing in a varying electromagnetic field is not possible here.
In addition to the aforementioned static test, it is also possible to determine the transponder-specific parameters dynamically, which is to say during the operation of a transponder. In the dynamic test, test equipment connected by wires is clamped to connections of the backscatter transponder. However, the clamped-on connecting lines act as antennas which likewise contribute to backscattering a part of the electromagnetic field transmitted by the base station. If the power of the electromagnetic signal radiated by the base station is relatively high, as in the case of passive transponders, the connecting lines, acting as antennas, must be expected to cause corresponding feedback effects on the circuitry of the analysis circuit. This leads to more or less altered and thus erroneous measurement results. Consequently, the results of this dynamic test in the electromagnetic field of the transponder system are not very reliable.
Thus, in current backscatter-based transponder systems there are no simple ways to reliably determine the aforementioned transponder-specific parameters in a UHF or microwave field. There exist only minimal tests where the transponder responds to a query signal transmitted by the base station by sending back data, which then only provide an indication of whether the modulator of the transponder is on or off. However, it is not possible to evaluate the transponder's performance, the effectiveness of its receiving device, or the like.